feat(cli): cph completions <shell> 子命令(clap_complete)

用 clap_complete 4 给 cph 生成 shell 补全脚本(bash/zsh/fish/powershell/elvish)。
clap ValueEnum 枚举 Shell;run_completions 从 Cli::command() 派生,自动跟随
子命令/flag 演进。用法:`cph completions zsh > ~/.zfunc/_cph`。

- 依赖经 `cargo add clap_complete@4 -p cph-cli` 添加(4.6.5,默认 features 无额外依赖)
- README 补补全安装一节

测试:全 workspace 49 passed。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/cph-cli/Cargo.toml

@@ -13,3 +13,4 @@ cph-check = { path = "../cph-check" }
 cph-diag = { workspace = true }
 cph-typst = { path = "../cph-typst" }
 clap = { version = "4", features = ["derive"] }
+clap_complete = "4"